1. Field of the Invention
The present invention relates to an elastic wave filter device utilizing a surface acoustic wave and an elastic boundary wave, and more particularly, the present invention relates to an elastic wave filter device configured by connecting a plurality of longitudinally coupled resonator elastic wave filters, and a duplexer including such an elastic wave filter device thereof.
2. Description of the Related Art
With cell phones, there has been demand for a reduction in the number of components, to realize reduction in size. Consequently, there has been demand for one component to have multiple functions. Balanced duplexers have been developed as one example of a component including such multiple functions. Balanced duplexers include a transmission-side filter connected to an antenna terminal, and a reception-side filter having a balanced-to-unbalanced conversion function. The reception-side filter has a balanced-to-unbalanced conversion function, so it is possible to omit a component configured to realize a balanced-to-unbalanced conversion function, i.e., a balun.
One example of a surface acoustic wave filter device which is used as such a reception-side filter has been disclosed in the following Japanese Unexamined Patent Application Publication No. 2002-290203.
FIG. 8 is a schematic plan view illustrating the electrode configuration of a surface acoustic wave filter device described in Japanese Unexamined Patent Application Publication No. 2002-290203.
A surface acoustic wave filter device 1001 is a longitudinally coupled resonator surface acoustic wave filter defining a balanced type filter. The surface acoustic wave filter device 1001 is configured by an electrode configuration shown in the drawing being formed on a piezoelectric substrate.
The surface acoustic wave filter device 1001 includes an unbalanced signal terminal 1002 defining an input terminal, and first and second balanced signal output terminals 1003 and 1004 defining output terminals. First and second longitudinally coupled resonator surface acoustic wave filters 1005 and 1006 are connected to the unbalanced signal terminal 1002. The first and second longitudinally coupled resonator surface acoustic wave filters 1005 and 1006 are longitudinally coupled resonator surface acoustic wave filters serving as 3-IDT type filters wherein the first longitudinally coupled resonator surface acoustic wave filter 1005 includes three IDTs 1005a to 1005c, and the second longitudinally coupled resonator surface acoustic wave filter 1006 includes three IDTs 1006a to 1006c. Note that reflectors 1005d and 1005e are disposed at both sides in the surface wave propagating direction of a region where the IDTs 1005a to 1005c are provided. With the longitudinally coupled resonator surface acoustic wave filter 1006 as well, reflectors 1006d and 1006e are provided in the same way.
First ends of the central IDTs 1005b and 1006b are connected to the unbalanced signal terminal 1002, and second of the IDTs 1005b and 1006b are connected to ground. First ends of the IDTs 1005a and 1005c at both sides of the longitudinally coupled resonator surface acoustic wave filter 1005 are connected to the first balanced signal terminal 1003, and second ends of the IDTs 1005a and 1005c are connected to ground. Similarly, first ends of the IDTs 1006a and 1006c at both sides of the second longitudinally coupled resonator surface acoustic wave filter 1006 are connected to the second balanced signal terminal 1004, and second ends of the IDTs 1006a and 1006c are connected to ground.
Here, the phase of an output signal to an input signal of the second longitudinally coupled resonator surface acoustic wave filter 1006 is different by about 180 degrees from the phase of an output signal to an input signal of the first longitudinally coupled resonator surface acoustic wave filter 1005, thereby realizing a balanced-to-unbalanced conversion function. The design parameters of the first longitudinally coupled resonator surface acoustic wave filter 1005 are assumed to be the same as those of the second longitudinally coupled resonator surface acoustic wave filter 1006 except for the above-mentioned phase of an output signal to an input signal.
With the surface acoustic wave filter device 1001 described in Japanese Unexamined Patent Application Publication No. 2002-290203, a balanced-to-unbalanced conversion function has been realized as described above, which enables the ratio of input-side impedance to output-side impedance to be set to about 1:2 to 1:4. For example, a balanced-type surface acoustic wave filter device is used as a reception-side filter at the RF stage of a cell phone, and an amplifier having balanced input is connected to the subsequent stage thereof. The input impedance of the amplifier connected to the subsequent stage is relatively high, so the surface acoustic wave filter device 1001 whose ratio of input-side impedance and output-side impedance is set to about 1:2 to 1:4 can be used appropriately.
On the other hand, as shown in the schematic plan view in FIG. 9, a balanced-type surface acoustic wave filter device 1101 including four longitudinally coupled resonator surface acoustic wave filters has been known. The surface acoustic wave filter device 1101 includes an unbalanced signal terminal 1102, and first and second balanced signal terminals 1103 and 1104 defining output terminals. First ends of second IDTs 1105b to 1108b positioned at the centers of first to fourth longitudinally coupled resonator surface acoustic wave filters are connected to the unbalanced signal terminal 1102, and second ends of the IDTs 1105b to 1108b are connected to ground.
The first and third IDTs 1105a, 1105c, 1106a, and 1106cat both sides of the first and second longitudinally coupled resonator surface acoustic wave filters 1105 and 1106 are connected to the first balanced signal terminal 1103. On the other hand, the first and third IDTs 1107a, 1107c, 1108a, and 1108c positioned at both sides of the central second IDTs 1107b and 1108b of the third and fourth longitudinally coupled resonator surface acoustic wave filters 1107 and 1108 are connected to the second balanced signal terminal 1104 as shown in the drawing.
Here, the phase of an output signal to an input signal of the third and fourth longitudinally coupled resonator surface acoustic wave filters 1107 and 1108 is different by about 180 degrees from the phase of an output signal to an input signal of the first and second longitudinally coupled resonator surface acoustic wave filters 1105 and 1106, and thus balanced output is taken out from the first and second balanced signal terminals 1103 and 1104.
However, with the surface acoustic wave filter device 1001 described in Japanese Unexamined Patent Application Publication No. 2002-290203, the crossing width of IDT electrodes is relatively long. Consequently, such as the case of the reception-side filter of cell-phone equipment for DCS or PCS, upon the surface acoustic wave filter being used as a band pass filter whose passband is about 2 GHz, the resistance of IDT electrode becomes high, i.e., ohmic loss increases, and consequently there is no choice other than increasing the insertion loss.
On the other hand, the surface acoustic wave filter device 1101 shown in FIG. 9 is a balanced-type surface acoustic wave filter device made up of four filter units and one stage, the crossing width of IDT electrodes can be reduced to about ½ as compared with the balanced-type surface acoustic wave filter device 1001 made up of two filter units and one stage. Accordingly, ohmic loss due to an IDT electrode can be reduced. However, the excitation of a bulk wave (SSBW) increases, ripple is readily generated within a passband, thus resulting in a problem wherein insertion loss increases.
Note that in recent years, instead of a surface acoustic wave filter device, an elastic boundary wave filter utilizing an elastic boundary wave has been disclosed wherein the simplification of a package configuration can be realized. With such an elastic boundary wave filter as well, as with the case of a surface acoustic wave filter, a configuration including a balanced-to-unbalanced conversion function, and also enabling the amount of out-of-band attenuation to be increased, has been strongly demanded.